According to the conventional water electrolysis system as described in JP 2010-43301 A, hydrogen generated by electrolyzing water is dehumidified alternately by two adsorption cylinders. When one of the adsorption cylinders is renewed, a part of the hydrogen dehumidified by the other adsorption cylinder, is introduced into the one adsorption cylinder so as to renew the one adsorption cylinder.
Further, according to the conventional water electrolysis system as described in JP 2007-231383 A, hydrogen discharged from a water electrolysis stack for generating high pressure hydrogen is stored in a hydrogen tank after its moisture is removed by an adsorption unit and, when the adsorption unit is renewed, the hydrogen in the hydrogen tank is fed to the adsorption unit after its pressure is reduced, so as to renew the adsorption unit.